Theoretical study of the acid-promoted hydrolysis of oxazolin-5-one: a microhydration model

The acid-promoted hydrolysis of 2,4,4-trimethyloxazolin-5-one (TMO) is studied employing the density functional theory (B3LYP) method in conjunction with the 6-31++G(d,p) basis set. Two types of reaction mechanism, N-protonated and O-protonated, are considered, involving protonation at the nitrogen and carbonyl oxygen of TMO to activate the C2 and C5 atoms, respectively, in favor of attack by water molecules. In the N-protonated pathway, the nucleophilic water molecule attacks the activated C2 atom, with a proton transfer from the water molecule to the oxygen atom attached to C2 and the fission of the C2-O bond, leading to a cis ring-opening product (N-acyl-alpha-amino isobutyric acid). While, in the O-protonated pathway, the nucleophilic water molecule attacks the activated carbonyl C5 atom, accompanied by a proton transfer from the water molecule toward the nitrogen atom of oxazole ring and the cleavage of C5-O bond; as a result, a corresponding trans product is generated. The water-assisted hydrolysis reactions are also examined together. A local microhydration model, in which an extra water molecule was added to obtain a continuous H-bond network around the reaction centers, was adopted to mimic the system for the two types of reaction processes. In addition, bulk solvent effect is introduced by use of the conductor-like polarizable continuum model (CPCM). Our computational results in kinetics and thermodynamics clearly manifest that the O-protonated pathway with the nucleophilic attack at the carbonyl C5 atom is more favorable than the N-protonated one, in nice agreement with the available experimental conclusion.